1. Field of the Invention
The present invention relates to an image processing apparatus suitable to a facsimile machine, electronic file, reader or digital copying machine.
2. Description of the Prior Art
In a prior art half-tone image processing system, an image signal level of each pixel is compared with a predetermined threshold and if the former is higher, black is reproduced, and if the former is lower, white is reproduced so that a pseudo-gray level is reproduced (systematic dither method). However, since this method has a much lower resolution power than that of a simple binary recording method (fixed threshold binary recording), the quality of reproduction of an image which requires a high resolution power, such as a character area, is lowered.
In order to resolve the above problem, in a method disclosed in Japanese Unexamined Pat. Publication No. 3374/1983 an image is divided into a plurality of blocks and the nature of each block, e.g., half-tone image area such as a photograph area, or a binary image area (line image area) such as a character area, is discriminated by the difference between maximum and minimum density levels of pixels in each block. This method is effective for the character or symbol area but when it is applied to a natural picture, the following problems are encountered.
(A) If a half-tone image area is misjudged to be a binary image area, the area to be represented by gray level is represented by white or black stripes because the half-tone area is processed by a single threshold. Accordingly, the image quality is degraded remarkably.
(B) If the character/symbol area has white or black background and fine black or white characters, that is, if the area has a high contrast, the above method is relatively effective, but if the area has a gray level background and black characters, the gray level area adjacent to the characters is misjudged to be a binary image area and the black or white stripes appear as in (A). Accordingly, the image quality is degraded.